1. Field of the Invention
The present invention relates to an organic electroluminescence device and a fabrication method thereof, and more particularly, to a top emission type organic electroluminescence device and a fabrication method thereof.
2. Discussion of the Related Art
In the field of flat panel display (FPD) devices, a liquid crystal display (LCD) device is widely utilized because it is lightweight and has low power consumption. However, the LCD device is a non-luminous display device and has technical limitations in brightness, contrast, viewing angle, and trend toward large size. For this reason, new flat panel display devices capable of overcoming these drawbacks have been actively developed.
One of the new flat panel display devices is an organic electroluminescence device. Since the organic electroluminescence device is a self-luminous display device, it has high contrast and wide viewing angle compared with the LCD device. Also, since the organic electroluminescence device does not require a backlight assembly, it is lightweight and slim. Moreover, the organic electroluminescence device is more efficient in power consumption.
The organic electroluminescence device is driven at a low DC voltage and has a rapid response time. Since all of the components of the organic electroluminescence device are formed of solid materials, they are durable against external impact. Also, the organic electroluminescence device can be manufactured in a wide temperature range, thereby saving manufacturing costs. In particular, the organic electroluminescence device can be easily fabricated through a deposition process and an encapsulation process. Therefore, the fabrication method and apparatus of the organic electroluminescence device are simpler than those of LCD device or plasma display panel (PDP) device. If the organic electroluminescence device is driven in an active matrix type, uniform brightness can be obtained even when a low current is applied. Accordingly, the organic electroluminescence device has advantages of low power consumption, high definition and large-sized screen.
FIG. 1 is a schematic sectional view of an active matrix organic electroluminescence device (AMOLED) that operates in a bottom emission type according to the related art. As shown in FIG. 1, first and second substrates 10 and 30 are arranged facing each other, and edge portions of the first and second substrates 10 and 30 are encapsulated by a seal pattern 40. A thin film transistor (TFT) T is formed on a transparent substrate 1 of the first substrate 10 in subpixel unit, and a first electrode 12 is connected to the TFT T. An organic electroluminescent layer 14 is formed on the TFT T and the first electrode 12 and also corresponds to the first electrode 12. The organic electroluminescent layer 14 contains light emission materials providing red, green and blue colored lights. A second electrode 16 is formed on the organic electroluminescent layer 14. The first and second electrodes 12 and 16 apply an electric field to the organic electroluminescent layer 14.
The seal pattern 40 separates the second electrode 16 from the second substrate 30 by a predetermined distance. An absorbent (not shown) and a translucent tape (not shown) may be further provided on an inner surface of the second substrate 30 such that the translucent tape bonds the absorbent to the second substrate 30 and the absorbent absorbs moisture introduced from an exterior.
In the bottom emission type structure, when the first electrode 12 and the second electrode 16 respectively serve as an anode and a cathode, the first electrode 12 is formed of a transparent conductive material and the second electrode 16 is formed of a metal having a low work function. In such an arrangement, the organic electroluminescent layer 14 includes a hole injection layer 14a, a hole transporting layer 14b, an emission layer 14c, and an electron transporting layer 14d, which are sequentially deposited on the first electrode 12. The emission layer 14c has red, green and blue color filters R, G, B in the sub-pixels. Thus, the array element and the organic electroluminescence diode are stacked on the same first substrate 10.
The bottom emission type organic electroluminescence device is fabricated by attaching a substrate, where the array element and the organic electroluminescence diode are formed, to another separate substrate provided for the encapsulation. In this case, the yield of the organic electroluminescence device is determined by the individual yields of the array element and the organic electroluminescence diode. Accordingly, the entire process yield is greatly restricted by the later process, namely, the process of forming the organic electroluminescence diode. For example, although excellent array elements are formed, as long as foreign particles or other factors cause defects in forming the organic electroluminescent layer using a thin film of about 1000 Å thick, the corresponding organic electroluminescence device will be determined to be defective. As a result, all of the expense and material costs spent in fabricating the non-defective array element will be lost, thereby degrading the yield.
While the bottom emission type organic electroluminescence device has high stability and high degree of freedom due to the encapsulation, it has limitations with regard to aperture ratio. Thus, it is difficult to apply the bottom emission type organic electroluminescence device to high-definition products. In the case of a top emission type organic electroluminescence device, the design of the TFTs is easy and the aperture ratio is high. In light of the overall lifetime of the product, it is advantageous to utilize the top emission type organic electroluminescence device. However, since the cathode is disposed on the organic electroluminescent layer, the selection of material is restricted. As a result, the transmittance is limited and the luminous efficiency is degraded.